Sustaining rotor for aircraft



Nov. 11, 1942. R H STANLEY 2,302,068v

SUSTAINING ROTOR FOR AIRCRAFT Filed Feb. 9, 1940 f I N VEN TOR! B Y, Wem7 A TTORNEYS.

Patented Nov. 17, 1.942

UNITED STATES PATENT OFFICEv SUSTAINING ROTOR FOR AIRCRAFT ApplicationFebruary 9, 1940, Serial No. 318,041

1 Claim.

This invention relates to sustaining rotors for aircraft, and is moreparticularly concerned with features of rotor blade construction, asfully described hereinafter.

Sustaining rotor blades are commonly constructed in such manner that atleast a large portion of the interior of the blade is hollow. Suchblades, moreover, are f substantial length, for instance from about toabout 25 feet, and rotate at a relatively high rate, for example from150 to 300 R. P. M.; and in the upper portion of the range of rotorvspeeds blade failures have been known to occur, for instance by tearingor rupturing oi" the external contour-defining material of the blade,the cause of which was not apparent.

I have found that when such a blade is rotated,

at high speed, pressures substantially above atmospheric are built up inthe outer portion of the blade, especially at the tip, as a result ofthe action of centrifugal force on the body or column of air within theblade, and I have further discovered that this built-up of pressure wasresponsible for such failure of the blade covering adjacent the tip end,and also in other cases for distortion of the sectional contour of theblade, especially toward the tip.

The present invention has in view overcoming the above difficulties.This is accomplished in accordance with the invention by providing avent or aperture in the blade covering in the outboard portion of theblade and preferably adjacent the tip. At the same time the inventioncontemplates substantial sealing of .the inboard end of the blade toprevent ingress of air. to its interior, from that end.

How the foregoing is accomplished, together with other objects andadvantages, will be more apparent from ther following description,referring to the accompanying drawing, in which- Figure 1 is an outlineperspective view of the underside of a rotor blade constructed inaccordance with the invention;

Figure 2 is a fragmentary enlarged sectional view of the blade of Figure1 taken in the region of the blade tip; and

Figure 3 is a longitudinal sectional view through the inner end of theblade spar and the associated root end fitting.

While the invention is applicable .to blades of a variety of differenttypes of construction and may be applied to any aircraft sustainingrotor blade having an internal hollow of substantial dimensionlengthwise of the blade, for purposes ing as applied to a blade of thetype built up on a. single spar, constituting the primary longitudinalstructural element, the spar carrying a multiplicity of spaced ribswhich, in turn, support sheet material, such as fabric, for defining theblade contour.

In Figure 1 the blade spar is indicated at 4, this spar being telescopedat its root end with a fitting 5 having spaced apertured lugs 6-6adapted to cooperate with a mounting pivot, by means of which the bladeis connected with the hub with freedom for swinging movement under theinfluence of flight forces. Any desired number and arrangement of pivotsmay, of course, be adopted, preferably including at least a flappingpivot providing freedom for swinging movement of the bladein a directiongenerally transverse its mean rotative path of travel and a drag pivotproviding for swinging movement of the blade generally Within its meanpath of travel. As shown in the drawing, the apertured lugs 6-6 areadapted to cooperate with a pivot of the latter type.

At intervals along the spar, a plurality of transverse ribs 1 areprovided defining a skeleton of the desired blade section and plan form.The trailing edges of the several ribs may be joined, if desired, by thetrailing edge Stringer 8 (see Figure 2).` In the region of both the rootand tip end portions of the blade, and also along the nose portion, theblade is preferably covered with plywood 9, and then fabric I0 isapplied over the entire blade structure, the fabric being doped inaccordance with accepted practice. A blade structure of the general typereferred to just above is more fully disclosed in copending applicationof Agnew E. Larsen and myself, Serial No. 309,586, filed December 16,1939.

In blades constructed as above, the ribs 1 are quite commonly apertured,as indicated at Il in Figure 2, so that all portions of the interior ofthe blade lengthwise thereof are in communication with each other. Evenwhere the ribs are not so apertured, it will be apparent thatsubstantial communication from end to end of the blade is usuallypresent by virtue lof clearance between the ribs and the spar where thelatter passes through the ribs, and at various points between the fabricand the ribs. In` such a. blade, for example of a length of about 2Ofeet and at a rotational speed of from 200 to 300 R. P. M., the internalpressure at the tip has'been found to-risel as high as 3.5 pounds persquare inch above atmospheric. 'I'his is sufficiently high to ofillustration the invention is shown in the drawdistort the blade contourby bulging of the fabric and even of plywood and sheet metal coveringsadjacent the blade tip, and actually to rupture the blade, especially inthose regions of the blade profile where the external pressure isreduced below atmospheric by the aerodynamic effects during operation ofthe blade.

With the foregoing in mind, in accordance with the present invention, avent aperture I2 is provided, preferably adjacent the tip of the bladeand on the underside thereof, closer to the trailing edge than to theleading edge. In addition to its positive functional advantages,location of the vent on the lower side of the blade tends to prevententrance of rain, etc. Positioning the Cil air from the interior of theblade and thereby results in substantial equalization of the internaland external pressures during rotation. It also, of course, permitsingress of air, when the rotor slows down, to balance the staticcondition.

At the same time, the invention contemplates substantial sealing of theinner end of the blade covering around the spar 4 or the blade rootIltting 5. This may be accomplished by use of an inner end closure ribor the like I5 having a. tight t or some sealing means applied aroundthe root end fitting 5, and to which the adjacent edges of the fabricare carefully doped. Care should also be taken in fabrication of theblade to make sure that the fabric throughout all of the inboard portionof the blade is carefully applied and doped in place so as to preventingress of air under the action of centrifugal force during rotation ofthe rotor. i

By the foregoing means provision is made for venting the interior bladehollow adjacent the outer end of the blade, and continuous circulationof air lengthwise through the bladevand out of the vent is at the sametime prevented. If this circulation or pumping" action takes place (asit would in the event of a substantial aperture in an inboard portion ofthe blade) certain undesirable pressure conditions are likely to be setup in the outboard portion of the blade notwithstanding the presence ofthe vent.

With this latter point in mind, I prefer also to plug or otherwise closethe inner end of the spar 4, as indicated at i6 in Figure 3. Anysuitable light plug, such as a cork, may serve this purpose, and therebyflow of air outwardly through the spar and into the tip of the blade,under the influence of centrifugal force, is prevented.

I have found that with a blade of the length and rotational speedmentioned above, a vent opening of about diameter will reduce theinternal pressure sufficiently to avoid substantial distortion andeliminate danger of failure, p ovided such opening is located withinabout tte outer one-third of the blade length. As ln cated above,however, the vent is preferably placed closely adjacent to the tip,although .care must be taken not to locate the vent outboard of anyimperforate rib which is so tightly sealed With the spar and the bladecovering as to impair the venting of air from the remainder of theblade. y I

Itwill be apparent that the invention is also applicable to metal orplastic blades of similar or other skeleton structure, so long as aninterior hollow is present of substantial dimension lengthwise of theblade.

I claim:

For an aircraft sustaining rotor, an elongated blade of airfoil sectioncomprising a longitudinally extending strength structure for mountingthe blade and sheet material carried thereby and dening the airfoilcontour of the blade and further dening an internal hollow extendedlongitudinally of the blade throughout at least a major portion of thelength of the blade, and the sheet material being subject to possibledeformation or rupture upon substantial build-up of air pressure in saidhollow by the centrifugal force of high speed rotation of the blade, anouter end portion of said hollow having venting means adapted to relieveair pressure therein built up by the centrifugal force of rotation, andthe inner end portion of said hollow being suiciently sealed to avoidbuild-upof a pressure suilciently to materially distort the airfoilsection or to rupture the blade despite the venting;

PAUL H. STANLEY.

